Don't overwrite empty lines

Fixes https://github.com/thatsmydoing/typingfreaks/issues/19

Previously, empty lines would not propagate down but just accept any
changes pushed from earlier lines. This is inconsistent with how a text
editor would normally behave with regards to the lines so we adjust it
here to match.

src/editor/lyrics.ts

@@ -316,7 +316,10 @@ export class LineEditor {
       this.adjustTimeInput();
     }
 
-    if (current === '') {
+    const isLastLine = part === 'time'
+      ? this.nextLine === undefined
+      : this.nextLine === this.container.lastLine;
+    if (current === '' && isLastLine) {
       return;
     }